A process is disclosed in DE 28 25 283 for the deposition welding of metals for repair of typical gas turbine drive mechanism parts of alloys based on Fe, Co or Ni with the use of a compatible filler wire. Such repair processes have the disadvantage that they form a polycrystalline region in the single-crystal composition of the drive parts which reduces the strength properties thereof.
A process and apparatus are disclosed in U.S. Pat. No. 3,857,436 for production of single-crystal structural parts by means of a single-crystal seed crystal. The repair of a single-crystal blade by this process and apparatus is substantially identical to a new production of the blade, since the single-crystal solidification proceeds from a seed crystal of small cross section relative to the blade cross section and reaches the full blade extent only after a transition region. A disadvantage is that the residual seed crystal and the transition region must be separated after the production of the blade. Further, this process and apparatus have the disadvantage that the seed crystal surface is subjected to an aggressive atmosphere during the entire cleaning and warmup time of the casting mold, due to the vaporization of volatile compounds, which increases the risk of polycrystalline growth, slip formation, and a high concentration of crystal defects, at least in the initial phase of crystal growth. Over and above this, it must be considered that no defect-free seed crystals are available and seed crystals which are rich in defects disadvantageously tend toward polycrystalline growth.